1. Field of the Invention
This invention relates to a highly selective method of producing formaldehyde, HCHO, by the partial oxidation of methane using a special type of catalyst.
2. Description of the Previously Published Art
The major commercial process to make formaldehyde is from methanol. This involves the steam reforming of methane with high grade heat to convert the methane into syn gas. The syn gas is reacted to form methanol while giving off low grade heat. Then the methanol is oxidized to formaldehyde while giving off additional low grade heat. This synthesis procedure requires multiple steps and it involves poor energy usage.
In the past a small fraction of formaldehyde was made by the partial oxidation of lower petroleum hydrocarbons which involved the partial oxidation of the hydrocarbon gas with air or oxygen under pressure, followed by rapid cooling, condensation, and absorption of the products in water to give a crude solution which must then be refined to separate formaldehyde from the other reaction products, such as methanol, acetaldehyde, propyl alcohol, propionaldehyde, and organic acids. Formaldehyde is isolated as a dilute solution, which must be concentrated to market strength. Propane and butane are the basic hydrocarbon raw materials for formaldehyde. Products manufactured by oxidation of propane and butane include formaldehyde, acetaldehyde, acetone, propionaldehyde, methanol, n-propyl alcohol, isopropyl alcohol, and butyl alcohols.
The German Offenlegungsschrift No. 2,404,738 to Bayer discloses oxidizing methane to formaldehyde by using many different types of metal oxides which may be placed on many different types of supports. The metal oxides are in Groups V, VI and/or VII of the Periodic Table. Among those listed are oxides of vanadium, niobium, tantalum, chromium, uranium, molybdenum, tungsten, manganese, technetium and rhenium, mixtures of these oxides with each other, and mixtures of these oxides with other oxides such as silica, alumina, iron oxide, calcium oxide, magnesia, sodium oxide or potassium oxide.
This reference is not helpful in finding an optimum methane conversion catalyst. It provides no attention as to the criticality of the support. It groups silica with alumina and other metal oxides yet, as was shown in U.S. Pat. No. 4,607,127, example 5, when alumina was used as a support for molybdena, it was not as effective as silica. Alumina is thought to interact with formaldehyde at high temperatures and, therefore, is also likely to be an ineffective support fo vanadium oxide.
Furthermore the Bayer reference provides no attention to the criticality of having a low sodium concentration. Sodium is present in most forms of silica that are synthesized in aqueous media because the conventional starting materials contain sodium. Offenlegungschrift No. 2404738 goes so far as to specify that up to 5 wt % Na.sub.2 O may be present in the working catalyst. As will be shown in Example 3, sodium in these concentrations would have a profoundly deleterious effect on selectivity to formaldehyde in the partial oxidation of methane.
U.S. Pat. No. 3,996,294 to Imre et al and assigned to Bayer discloses the use of a silica catalyst which does not contain vanadium to produce formaldehyde from methane. As will be shown in the examples, a catalyst consisting purely of silica, such as Cabosil (a product of Cabot Corporation), exhibits a lower yield of formaldehyde than is obtained with the catalyst of the present invention. Some of the examples in the Imre et al patent employ catalysts made substantially of silica along with small amounts of other metal oxides. There is no example given of using vanadium oxide, although, as in the companion German Offenlegungsschrift, there is a broad list of metal oxides which can be used, including oxides of aluminum, iron, vanadium, molybdenum, tungsten, calcium, magnesium, sodium or potassium with a specific reference that up to 5% of Na.sub.2 O can be used.
Japanese Patent Publication No. 58-92630 discloses a SiO.sub.2 -V.sub.2 O.sub.5 catalyst, using N.sub.2 O as an oxidant. A similar catalyst is reported by K. J. Zhen et al in J. Catalysis, 94 (1985) 501-507. The use of N.sub.2 O as an oxidant is prohibitively expensive.
U.S. Pat. No. 4,607,127 to Spencer discloses a MoO.sub.3 -SiO.sub.2 catalyst with low sodium concentration, which can be used to oxidize methane to formaldehyde using oxygen. This catalyst is less active than that of the present invention, as is shown in Comparison Example 3. The U.S. Pat. No. 4,607,127 patent makes no reference to oxides other than MoO.sub.3 or SiO.sub.2 as being active catalysts for this reaction.
3. Objects of the Invention
It is an object of this invention to produce formaldehyde from methane without the production of a large number of by-products so as to avoid the attendant separation problems.
It is a further object of this invention to partially oxidize methane to form formaldehyde.
It is a further object of this invention to convert methane to formaldehyde with high selectivity.
It is a further object of this invention to obtain a methane conversion catalyst for use in the present process which converts methane to formaldehyde with high selectivity.
It is a further object of this invention to produce a methane conversion catalyst having a formaldehyde yield of greater than or equal to 1% when operating at a space velocity of 5000 hr.sup.-1 (NTP), at a temperature of 600.degree. C. and atmospheric pressure.
These and further objects will become apparent as the description of the invention proceeds.